Solder bearing lead and method of fabrication

ABSTRACT

A solder-bearing lead, and methods of fabricating and using it, where the lead is formed with a solder-retaining portion having a cross-sectional profile with undercuts and a tip, preferably in a V-shape, where solder surrounds said solder-retaining portion and undercuts, and extends outwardly substantially no farther than said tip.

FIELD OF THE INVENTION

This invention relates to solder-bearing leads for attachment toconductive pads on substrates, such as printed circuit boards, chipcarriers, integrated circuits, or the like.

DESCRIPTION OF RELATED ART AND BACKGROUND OF THE INVENTION

In the electronic equipment industry, it is common to provide leads forconnection substrates, such as printed circuit boards, chip carriers,and the like, by soldering the leads to conductive pads on thesubstrate. See, for example, U.S. Pat. No. 4,728,305 issued Mar. 1, 1988and U.S. Pat. No. 5,030,144 issued Jul. 9, 1991. Such substratescommonly have a row of conductive pads or contact areas along one ormore edges, for connection to an array of generally parallel leadsspaced correspondingly to the pad spacing. The leads are commonlyconnected at one end to a longitudinally extending carrier strip, sothat a section of the carrier strip having a ganged array of leadsextending from it, may be applied to the row of pads along one edge ofthe substitute for simultaneously soldering the ganged array of leads tothe row of pads.

The trend in this industry has been towards greater and greaterminiaturization. As a result, the spacing between conductive pads of asubstrate has been progressively reduced. While a pitch of 0.100 inch(that is, 10 pads to the inch) has been common, the industry is movingmore and more toward smaller pitches, such as 0.075 or 0.050 inch oreven less. This requires a corresponding reduction of spacing and sizeof leads which are to be connected to such conductive pads.

In satisfying such needs for reduced spacing between leads, sufficientspacing must be nevertheless provided between the leads to preventinadvertent contact or arcing between adjacent leads. This must be donewithout sacrificing accuracy in manufacture or impeding rapid assemblyof multiple leads. A particular problem arises where the leads aresolder-bearing, since in some instances the arrangement for holding thesolder to the lead may encroach into the space between leads, therebylimiting how closely the leads may be spaced. See for example said U.S.Pat. No. 4,728,305.

Accordingly, a need exists for solder-bearing leads of reduced size andspacing which are effectively applicable to accommodate reduced size andspacing of substrate conductive pads, but sufficiently spaced to preventarcing, and to provide methods of manufacturing such leads withoutsacrificing accuracy.

SUMMARY OF THE INVENTION

The present invention overcomes some of the problems and deficiencies ofthe prior art by providing solder-bearing leads of reduced size andspacing to accommodate the desired reductions in spacing of contact padson substrates. Closer spacing of leads is attainable by eliminatinglateral extensions of the leads which may intrude into the space betweenleads, and thus limit permissible spacing of such leads. In addition, Atthe same time, provision is made for effectively retaining one or moresolder beads or slugs on each lead, without unduly intruding into thespace between leads to such an extent as to prevent close spacing ofleads. In addition, provision is made to maintain direct metal-to-metalcontact between lead and pad during soldering so that re-flow of thesolder will not disturb the relation between lead and pad, and willprovide a good solder joint.

The solder-bearing lead of the invention can be used, among other ways,as an edge clip for mounting to the edge of a substrate, or for surfacemounting on a substrate. It can advantageously be made by automaticprogressive stamping of blanks at high speed, in a continuous gangedarray of leads held on one or more carrier strips, suitable for rollingup on a reel for convenience of transportation and ease of use inassembly to a chip carrier or other substrate.

The objects, features, and advantages of the present invention will bemore completely understood by referring to the following detaileddescription of presently preferred embodiments of the present invention,taken with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a portion of a continuous strip having a gangedarray of lead blanks, each integral with a carrier strip, and useful inthe present invention.

FIG. 2 is a similar plan view of the arrangement of FIG. 1, aftersubsequent forming operations at the intended solder location.

FIG. 3 is an enlarged fragmentary plan view of a portion of one leadshown in FIG. 2.

FIG. 4 is a cross-sectional view of the structure of FIG. 3, taken alongline 4--4 thereof,

FIG. 5 is a view similar to FIG. 2, after placing a solder wire across asection of the array of leads.

FIG. 6 is a plan view similar to FIG. 5 after the solder wire is cut andformed around the solder location.

FIG. 7 is an enlarged fragmentary transverse cross-sectional view of onelead of FIG. 6 viewed along line 7--7 thereof.

FIG. 8 is an enlarged fragmentary longitudinal cross-sectional view of alead of FIG. 6, viewed along line 8--8 thereof.

FIG. 9 is an enlarged fragmentary side elevation view of the lead ofFIGS. 6 to 8.

FIG. 10 is a plan view of a fragment of a continuous strip of gangedleads according to a modified form of the invention, at an early stageof fabrication, with excess portions cut away to leave an array of leadblanks secured to a carrier strip.

FIG. 11 shows a plan view similar to FIG. 10 after a further fabricationstep.

FIG. 12 is an enlarged cross-section view of a lead of FIG. 10, showinghow the cross-section is reformed for accepting a solder bead.

FIG. 13 shows a plan view similar to FIG. 11 of a further stage in thefabrication of the leads according to the present invention, by laying asolder wire across a section of the array of leads of FIG. 11.

FIG. 14 is an enlarged perspective view showing the relation of thesolder wire to a lead in FIG. 13.

FIG. 15 is a plan view of a section of the array of leads of FIG. 13,after the solder wire is cut and formed around the solder location, ofthe lead.

FIG. 16 is an enlarged transverse cross-sectional view of a lead of FIG.15 viewed along line 16--16 thereof.

FIG. 17 is an enlarged longitudinal cross-sectional view of the solderbearing portion of a lead of FIG. 15 or 16, viewed along line 17--17 ofFIG. 16.

FIG. 18 is an enlarged fragmentary side elevation view of lead of FIGS.15 to 17.

FIG. 19 is a side elevation view partly in section of a substrate onwhich is mounted an edge clip having on one leg a solder mass formed asin FIGS. 1 to 9, preliminary to re-flow of the solder.

FIG. 20 is a side elevation view similar to FIG. 19, but having twosolder masses, one on each face of the substrate.

FIG. 21 is a side elevation view of a substrate positioned on anS-shaped lead of FIGS. 1 to 9.

FIG. 22 is a side elevation view of a substrate with a lead of FIGS. 1to 9 positioned to be surface mounted thereon.

FIG. 23 is a side elevation view similar to FIG. 20, with provision forextra security for retaining the solder mass on its lead.

FIG. 24 is a side elevation view similar to FIG. 20 showing a lead as inFIGS. 10 to 18 positioned on a substrate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, where like reference numerals representcorresponding parts in the several views, FIGS. 1 to 9 show a first formof the invention. A continuous flat uniform slightly resilient strip ofelectrically conductive material (such as beryllium copper, brass orphosphor bronze, any of which may be pre-plated with solder or tin) ofuniform thickness and width is passed through a progressive stampingmachine to cut out extraneous materials, leaving a stamped stripillustrated in FIG. 1, having a sequence of side-by-side individual leadblanks 10 joined integrally at one end to a carrier strip 12. As iscustomary, the carrier strip 12 may include periodically spaced holes 14for indexing the strip through automatic stamping apparatus. In thisform of the invention, at the position where a solder bead is to be heldon the lead, each lead blank 10 has a reduced-width section 16. Thissection 16 is located along lead 10 at a position, depending upon theultimate configuration of lead 10, where it is desired to retain asolder mass or bead. This reduced-width section 16 is joined to the mainlead blank 10 by shoulders 18 at each end of section 16, which, as willbe seen below, form stops or holding means to prevent longitudinalmovement of the solder bead along the lead 10. At an appropriateposition along each lead 10, it is provided with a transverse scoring ornotch 19 which permits the lead 10 to be broken or cut away from thecarrier strip 12.

According to the invention, the reduced-width section 16 is formed, asby stamping or pressing along its center line 21, into a generallyV-shaped cross-section 20 illustrated in FIGS. 2 to 4. The apex 22 ofeach V structure 20 projects slightly below the under surface of itslead 10 as shown most clearly in FIG. 4.

In a subsequent step of the process of fabrication, the lead is bentslightly, to form an obtuse angle, at the position of the reduced-widthsection. Then a solder wire or rod 24 is laid across a number of leadsat the locations of their reduced-width sections 16, as illustrated inFIG. 5. Preferably the solder wire 24 contains a rosin core forming asolder flux. In a subsequent operation, the solder wire 24 is severed oneach side of the reduced-width section 20 and then formed around theV-shaped section 20 as illustrated in FIGS. 6 to 9 to form a solderbead. It will be recognized that the solder material is highly malleableand therefore lends itself readily to such forming around the V-shapedsection 20. As shown in FIG. 7, the width of the formed solder mass orbead 26 transverse to the lead 10, may be made substantially equal to oronly slightly wider than the width of lead 10, so that the 26 encroachesonly slightly or not at all into the space between successive leads 10,thereby permitting the leads to be closely spaced to conform to thesubstrate conductive pad spacing, which therefore may be substantiallyless than the conventional spacing of 0.050 inch. By appropriatelyselecting the diameter or cross-section of the solder wire 24, theheight of the solder bead 26 may be made as large as necessary toprovide the correct amount of solder for the proper solder joint to bemade between each lead and its corresponding conductive pad.

As seen in FIG. 7, the solder 26 is caused to surround the V-shapedsection 20 while not covering the apex 22. Also, the undercut providedby sloping sides of the V-shaped cross-section provides an interlockwith the solder mass to prevent its accidental removal transversely ofthe lead 10, while the shoulders 18 serve to prevent longitudinalmovement of the solder bead along the lead 10, so that the bead isretained reliably at the desired position.

One form which a completed lead may take is shown in FIG. 19, having agenerally S-shaped configuration. This configuration is formed by threegenerally parallel legs 28, 30, 32, with legs 28 and 30 joined by anappropriately shaped joining section 34, preferably curved in whole orin part, and legs 30 and 32 are also joined by a similar section 36.This configuration is adapted to receive a substrate 38 between upperleg 28 and central leg 32 of the S-shaped configuration, therebyproviding a resilient clip mounting for the substrate 38 and reducingpotentially harmful effects on the soldered joint which may be createdby shock, vibration or differential thermal expansion.

The substrate 38 (which may be a chip carrier or an integrated circuitor a printed circuit board or the like) carries along one or more edgesthe usual linear array of conductive pads or contacts 40. In assemblingclip leads such as in FIG. 19 to the substrate, a section of the carrierstrip having a number of ganged leads 10 equal to the number ofconductive pads 40 along an edge of the substrate, is cut off, and theclip-like arrangement of FIG. 19 is mounted on the substrate, with thesolder mass 26 of each lead in contact with a corresponding substratecontact 40. The apex 22 of the lead's V-shaped section 20 then makesdirect metal-to-metal contact with the conductive pad 40, with no solderin between. This provides the advantage that the lead 10 and pad 40 arereadily maintained in desired fixed relation during soldering, which isnot altered by subsequent re-flow or melting of the solder. The slopedouter end of the V-shaped section 26 and the adjoining flat portion 39of lead 10 serve as a ramp or slope to facilitate entry of the substrate38 between the legs 28 and 30.

While FIG. 19 shows a single solder bead, on one leg 28, it will beunderstood that one or more additional solder beads may be mounted onlead 10, in a fashion similar to bead 26. For example, a second similarsolder mass may be formed on leg 30, as shown in FIG. 20, so as topermit soldering each lead 10 to conductive pads 44 on opposite faces ofthe substrate 38.

The leg 32 of the arrangement of FIG. 19 and 20 may be surface-mountedon a second substrate by a similar solder retaining arrangement (or anyother soldering arrangement) on leg 32, or leg 32 may be extended toform a terminal for connection to order apparatus. For example, leg 32may be bent at right angles to the position shown, and shaped to extendthrough a hole in a second substrate for perpendicular mounting andsoldering to the second substrate.

FIG. 21 shows another arrangement, with lead 10 formed generallysimilarly to that of FIG. 19, but arranged to be soldered to aconductive pad 40 with the substrate on top of the lead 10 and solderbead 26.

Instead of forming a clip as in FIG. 19 or 20, the lead of the inventionmay be directly surface-mounted on a substrate by soldering thesolder-bearing section to a substrate as shown in FIG. 22. The other end10a of lead 10 may have any desired configuration, as required byapparatus to which it is to be connected.

In some instances, it may be desirable to give added security forretention of the solder on the lead, in any of the above-described formsof the invention. This may be done as shown in FIG. 23, which shows anedge clip similar to FIG. 20. Here, the lead portion 10b extendinglongitudinally outward from the solder bead 26 is bent over and closelyjuxtaposed to the solder 26 to further prevent movement of the soldermass away from lead 10. In addition, lower leg 32 of the lead 10 is bentto be close to the lower solder mass 26a, to aid in retaining it on thelead, if needed.

While the undercuts for retaining the solder mass on the lead are shownin FIGS. 1 to 9 as formed by bending the lead body into a V-shape abouta longitudinal axis, it will be understood that other ways of providingthe undercuts and apex may be used. Thus, FIGS. 10 to 18 illustratestages in the fabrication of a modified form of the present invention.FIG. 10 shows the lead blanks formed by cutting out suitable portions ofa continuous strip of conductive material to leave an array of flatleads 50 of uniform width attached at one end to and integral with thecarrier strip 12. Instead of forming the V-shaped section as shown inFIGS. 1 to 9, at the location of each lead where the solder lead is tobe applied the lead is coined into a V-shape as shown in FIGS. 11 and12. This is done by applying pressure to the somewhat malleable lead,without removing material. The material formerly in the corners 54 ofthe leads 50 is moved away by this operation so that the width of thecoined V-shaped section will be slightly larger than the original widthof the leads 50, as seen in FIG. 12, or the upper surface of the leadmay bulge upward. Shoulders indicated at 56 in FIG. 14 are formed at theends of the coined section 58. While shown as squared off, theseshoulders may be somewhat rounded or tapered. After thus forming theleads with a Section 52 having a V-shaped cross-section, again a solderwire 20, preferably rosin-cored, is laid transversely of the leads 50 atthe position of the V-shaped section 52, as shown in FIG. 13 and inperspective view in FIG. 14. The solder material is then again moldedaround the V-shaped coined section 56, as seen in transversecross-section in FIG. 16 and in longitudinal cross-section in FIG. 17.Here again, as in the preceding form, the solder 20 does not extendsubstantially beyond apex 22 of the V, to allow the apex 22 to be placedin direct contact with a mating conductive pad.

By forming the solder 20 around the lead 50 at the coined V-shapedsection 52, the solder bead 20 is retained against transverse movementby the undercut provided by the inwardly sloping sides of the V-shapedsection, and the shoulder 56 at each end of the V-shaped section retainsthe solder bead against longitudinal movement. Again, the width of thesolder bead 20 may be made equal to or only slightly wider than thewidth of the lead 10 or 50, so that in this arrangement as well as inthe previously described one, leads may be relatively close together,since little if any portion of the leads extends into the space betweenthe leads.

Thus, by the simple process of coining the V-shaped section instead ofbending the lead into a V-shaped section, a similar profile is obtained,useful in the same way as described above, including FIGS. 19 to 23. Thesolder-retaining arrangement described above with respect to FIGS. 10 to18 may thus be used in place of the arrangements of FIGS. 1 to 9described with respect to FIGS. 19 to 23. FIG. 24 shows, in a fashionsimilar to FIG. 22, how each lead 50 with a solder bead 26 formed abouta coined V-shaped section 52 may be positioned relative to a conductivepad 40 of a substrate 38, for soldering. As before, the lead body 50 maybe formed as described into a variety of shapes known to the art,including edge clips or for surface-mounting, shown in FIGS. 19 to 23.

In assembling the leads to a chip carrier or other substrate having arow of contact pads along one or more edges, a section of the carrierstrip is cut off, the section including a number of leads correspondingto the number and spacing of contacts on the substrate. While heldtogether by the carrier strip, the leads are positioned in contact withthe respective pads, so that the apex of the undercut portion of eachlead makes direct contact with the corresponding pad. The ganged arrayof leads may be held against the pads by the resilience of the cliparrangement when a configuration as in FIG. 19 is used. Otherwise theleads and pads may be held in position by suitable fixtures. The solderis then re-flowed, and then cooled, which joins each lead to a pad bothelectrically and mechanically. Thereafter the carrier strip is severedor broken away, as by the notch construction 19. The other ends of theleads may then be connected suitably to other devices.

By way of example, the leads of the present invention may be spaced moreclosely even then the present general minimum pitch of 0.050 inch (20 tothe inch). As an illustration (but not limitation) of the miniaturedimensions attainable with the present invention, each lead may have awidth of 0.010 to 0.015 inch, with a thickness of 0.005 to 0.008 inch.The reduced-width section 16 of FIGS. 1 to 9 may be between 0.005 and0.008 inch wide and 0.010 to 0.012 inch long. The apex 22 of theV-shaped section 20 in FIGS. 1 to 9 may project below the lead undersurface by 0.001 to 0.003 inch. Corresponding dimensions may be used forthe form of FIGS. 10 to 18. Therefore, pitches of as small as 0.040 to0.025 inch (25 to 40 leads per inch) may be provided without creatingshort circuits between the leads or after soldering between theconductive pads to which they are soldered.

It will be understood that, while the V-shaped profile of thesolder-retaining section is a simply attainable and a desirable form,other profiles may be used. The present invention provides forinterlocking or retaining the solder bead to the lead without preventingmetal-to-metal contact between lead and its corresponding conductive pador contact. Thus, without departing from essential aspects of theinvention, the V-shaped profile may be modified to have a curved form ora stepped form. Also the solder-retaining section profile may be formedin other ways, as by suitably removing materials from the lower cornersof the edges of the leads to provide the desired undercut, and apex ortip.

It will be understood that the invention is not to be limited by thespecific embodiments shown in the drawings and described in the abovedescription, which are given by way of example and not of limitation,but only in accordance with the scope of the appended claims.

What is claimed is:
 1. A connector arrangement for a substrate having a row of conductive pads, comprising:a carrier strip of electrically conductive material of substantially uniform thickness, a plurality of elongated conductive leads, each integrally connected to said carrier strip at one end of the lead, said leads being parallel and spaced corresponding to said pads, each lead comprising: a generally flat body portion of substantially uniform width and thickness, with an upper surface and a lower surface, a longitudinally extending solder-bearing portion at a predetermined position along said lead adjoining said flat body portion and having a cross-section with undercuts at the edges of said solder-bearing portion, an interior portion of said cross-section having a tip extending at least to the same extent as the lower surface of said adjoining flat body portion, a shoulder between said solder-bearing portion and said flat body portion at an end of said solder-bearing portion, and a solder mass mechanically formed about said solder-bearing portion, and extending around said undercuts and substantially to said tip, said solder mass formed at a temperature below the melting point of said solder mass, whereby said solder mass is prevented from longitudinal movement by said shoulder and from transverse movement by said undercuts, and said lead may be soldered to a corresponding pad by placing said tip in contact with said pad and reflowing said solder mass.
 2. A connector arrangement as in claim 1, in whichsaid solder-bearing portion has a generally V-shaped profile in cross-section, with the vertex of said V-shaped profile extending transversely at least to the lower surface of said flat body portion.
 3. A connector arrangement as in claim 1, where said tip extends beyond the under surface of said flat body portion.
 4. A connector arrangement as in claim 1, wherein said solder-bearing portion is narrower than said lead body portion and is bent about a longitudinal axis to form said undercuts and said tip.
 5. A connector arrangement as in claim 2, wherein said vertex is substantially co-planar with said lead body lower surface and said solder mass.
 6. A connector arrangement as in claim 2, wherein said vertex extends beyond said lead body lower surface and solder mass.
 7. A solder-bearing lead for connection to a substrate conductive pad comprisingan elongated lead body of electrically conductive material, said lead body having substantially uniform width and thickness with an upper surface and a lower surface, said lead body having a longitudinally extending solder-bearing portion at a predetermined position along said lead body and adjoining a uniform thickness section of said lead body, said solder-bearing portion having a cross-section with undercuts at the edges thereof and an interior portion with a tip extending at least to the same extent as said lead body lower surface, said lead having a shoulder between said uniform thickness body section and said solder-bearing portion, and a solder mass mechanically formed about said solder-bearing lead portion and extending around said undercuts, but not extending substantially beyond said tip, said solder mass formed at a temperature below the melting point of said solder mass, whereby said solder mass is prevented from longitudinal movement by said shoulder and from transverse movement by said undercuts, and said lead may be soldered to a corresponding pad by placing said tip in contact with said pad and reflowing said solder mass.
 8. A solder-bearing lead as in claim 7, wherein said solder-bearing lead portion has a V-shaped cross-section, said undercuts being provided by the sloping sides of said V and said tip being the vertex of said V.
 9. A lead as in claim 8, wherein said vertex extends beyond said lead body lower surface. 